Fiber-optic current sensors rely on the magneto-optic Faraday effect in an optical fiber that is coiled around the current conductor. The current-induced magnetic field generates circular birefringence in the optical fiber that is proportional to the applied magnetic field. A preferred arrangement employs a reflector at the sensing fiber's far end so that the light coupled into the fiber performs a round trip in the fiber coil. Commonly, left and right circularly polarized light waves are injected into the sensing fiber which are generated from two orthogonal linearly polarized light waves by a fiber optic phase retarder spliced to the sensing fiber and acting as quarter-wave retarder (QWR), as described in reference [1]. After the round trip through the fiber coil the two circular waves have accumulated a relative phase delay proportional to the applied current as a result of the circular birefringence in the fiber. This phase delay is proportional to the number of fiber windings around the current conductor, the applied electrical current, and the Verdet constant V(T, λ) of the fiber: The Verdet constant is material-, temperature-, and wavelength-dependent.
As an alternative the sensor may be designed as a Sagnac-type interferometer with quarter-wave retarders (QWRs) at both sensing fiber ends and light waves of the same sense of circular polarization that are counter-propagating in the sensing fiber (see ref. [1]).
Further known are voltage or electric field sensors based on the Pockels effect (linear electro-optic effect) [21] or on the use of an optical fiber coupled to a piezo-electric material [16, 7]. In these sensors, birefringence induced by the electric field or force or anisotropic change in the refractive index of the material is used in the optic sensor to measure voltages or electric field strength.
High performance current sensors often use an interferometric technique based on non-reciprocal phase modulation as also applied in fiber gyroscopes in order to measure the optical phase shift, see e.g. ref. [2]. Integrated-optic phase modulators or piezo-electric modulators are employed. The technique provides, particularly in combination with closed-loop detection, high accuracy, good scale factor stability, and a linear response over a large range of magneto-optic phase shift. On the other hand the technique is relatively sophisticated and often requires polarization-maintaining (PM) fiber components and elaborate signal processing. Moreover, integrated-optic modulators are relatively expensive components.
By contrast simpler detection schemes employ passive optical components such as wave-plates and polarizers which convert the magneto-optic phase shift into a change of the transmitted optical power (as described for example in reference [3]). In order to make the sensor output independent of, e.g., variations of the light source power, such sensors often work with at least two detection channels. The optical power in the two channels varies with opposite phase (anti-phase) in response to the current to be measured. In principle, the difference of the two signals divided by their sum is proportional to the current and is independent of the source power. However, asymmetries in the two channels, such as different optical losses, influences of stress, and their variation over time and temperature, limit the achievable accuracy of this type of sensors. While the sensor accuracy may be sufficient for protection functions in high voltage substations (IEC accuracy class 5P demands an accuracy to within ±1% at the rated current), the accuracy typically is insufficient for electricity metering; the IEC metering class 0.2 for example demands an accuracy to within 0.2% at the rated current (reference [4]).
U.S. Pat. No. 5,895,912 discloses a polarimetric AC sensor in which DC signal components are used to intensity-normalize the AC measurement signals.
It is therefore an object of the invention to provide optical sensors of the above kind, such as magnetic field sensors or fiber-optic current sensors (FOCSs), and related methods that increase the accuracy of such sensors even when using passive optical components instead of actively phase modulating components to detect a relative phase shift between light waves.